1. Field of the Invention
The present invention relates to a method and apparatus for image forming preferably applicable for a printer, facsimile machine, copier, multi-functional machine, and so forth. More particularly, the present invention relates to an image forming apparatus that can effectively adjust respective phases of a plurality of image bearing members to become correlated in phase relationships so that a printing operation can instantly be started.
2. Discussion of the Background Art
Some background image forming apparatuses have a tandem-type structure in which a plurality of image forming mechanisms respectively form toner color images corresponding to different color components according to respective image data.
When a recording medium passes the plurality of respective image forming mechanisms, respective timings of passing the plurality of respective image forming mechanisms corresponding to respective color components are different from each other. Therefore, a write start timing in a sub-scanning direction in the respective image forming mechanisms are adjusted by a light beam emission start timing. That is, when a light beam deflected according to the image data of the corresponding color toner image is emitted from an optical writing unit, the start timing of emitting the laser beam is required to be controlled such that each single color toner image can properly be overlaid at a transfer position in the image forming mechanism onto the recording medium closely attached onto the sheet transfer belt.
However, another control other than the control of the write start timing is required to obtain a preferable transfer image. That is, a plurality of photoreceptors that ideally have a cross-sectional circular form and rotate around a center axis of the circular form may have eccentricity and deviate from the center axis of rotation, as shown in FIGS. 1A and 1B. The cause of the eccentricity or deviation is based on limitations of maintaining the manufacturing accuracy and/or the assembly accuracy of each photoreceptor, which is unavoidable.
When the rotation axis becomes eccentric, a circumferential speed of a photoreceptor 901 with respect to a transfer belt 902 may vary according to a rotational phase of the photoreceptor 901. That is, the circumferential speed of the photoreceptor 901 with respect to the transfer belt 902 may be different between a condition when a rotation radius of the photoreceptor 901 reaches its maximum value as shown in FIG. 1A and a condition when the rotation radius of the photoreceptor 901 reaches its minimum value as shown in FIG. 1B.
Therefore, when an image of lines of a laser beam is written onto a circumferential surface of the photoreceptor 901 as shown in FIG. 2A, the image of lines may be written at even intervals at constant rotations as shown in FIG. 2B. However, the circumferential speed may become different according to the rotational phase of the photoreceptor 901. Therefore, when transferred onto the recording medium, the image of lines may be written at uneven intervals caused by variations of the circumferential speed of the photoreceptor 901, as shown in FIG. 2C.
To compensate for the variation of the circumferential speed of the photoreceptor, a write timing can be varied according to the phase of the photoreceptor. For example, intervals of writing can be set smaller when the circumferential speed is in a fast phase, and conversely, intervals of writing can be set greater when the circumferential speed is in a slow phase. By performing the above-described operations, color shifts on an overlaid image due to rotational phase shifts of each photoreceptor may be prevented.
When a series of image forming operations, however, are performed without properly adjusting the relationship of phase in respective photoreceptors to a predetermined condition that is a reference of a phase correction of write timing, the overlaid (and printed) image may have color shifts caused by rotational phase shifts of a photoreceptor as shown in FIG. 3.
Thus, it is preferable that the phases of the respective photoreceptor are adjusted to a predetermined phase relationship prior to the start of the image forming operations start so as to obtain a preferable overlaid image.
To adjust the phases of the respective photoreceptor to a predetermined phase relationship, it is preferable to perform a phase detecting operation and a phase adjusting operation prior to a series of image forming operations. The phase detecting operation is performed to detect phases of the respective photoreceptor. The phase adjusting operation is performed to adjust the phases of the photoreceptor detected through the phase detecting operation to the predetermined phase relationship.
A phase detecting operation that is widely known is performed in the steps noted below.
When a print request from a user is received, an image forming apparatus starts to rotate a plurality of photoreceptors, and detects respective rotational phases of corresponding each of the plurality of photoreceptors. Thereby, phases of each photoreceptor can be detected.
In the phase detecting operation, a measurement start point detected by a photointerruptor and so forth is specified as an origin, and pulses generated with respect to rotation angles of the respective photoreceptors are adjusted by using a rotary encoder with an incremental method.
Further, the phase adjusting operation is performed by increasing or decreasing the rotational speeds of the respective photoconductors so that current rotational phases, which are detected values, of the respective photoreceptors and the predetermined phase relationship, which are target values, of the respective photoreceptors are compensated.
In the background art, the above-described phase detecting and adjusting operations are performed after a printing operation is requested by a user, and then the actual image forming operation is started to discharge the printouts out of the image forming apparatus.
The above-described phase detecting and adjusting operations of a photoreceptor, however, are performed during a period from a receipt of the print request to a production of printouts, that is, prior to a start of the actual image forming operation. Therefore, a period of time required to obtain the desired printouts may seem long to users.
Some techniques have shown a variety of ways to reduce the phase adjustment time.
However, these techniques have shown that the phase detecting and adjusting operations of the photoreceptors are performed during the period of time from the receipt of the print request to the start of the actual image forming operation. Even if the period of time required for the phase detecting and adjusting operations is reduced, these operations need to be performed before the start of the actual image forming operation. Therefore, the waiting time that is the operation period may still seem long for users.